All the way from R-300(ATI 9500) to R-360 (9800 xt) ATI has been able to ride this Chip through the Video Battlefield.
The R-420 (x800)is no different , same chip optimized to its limits and still performing well.
Nvidia on the other hand has made a major change with there new NV40 chip (6800) and for once a change for the better sense the FX dustbuster.Surely ATI has somthing up there sleave next year but for now the 6800 sieries is the card to buy.Preferably the Ulta line .
faster mean the game run very smooth althought not as pretty graphic than Nvidia.
I prefer the game can run smooth cosz while playing i dont have time to concentrate the graphiic.
I would choose NVIDIA as a professional, they just work faster.
I would choose ATI as a gamer, all high performance Radeon cards can handle it when you turn on AA or AF, NVIDIA graphics card performance drops so much with AA/AF enabled.
I, personally would prefer a ballance between performance and quality, I really love good framerates but I also love stunning graphics, I would get the 6800GT It is better quality graphics, and if you cant stand it being not quite as effecient with framerates, you can always overclock it.
here is a link to the first DOOM 3 bench mark. NVIDIA seems to have the lead but is it because they optimized their drivers for this particular game again. http://www2.hardocp.com/article.html?art=NjQy
You can find plenty of reviews, check Tom's hardware and Anand tech sites.